Thermoelectric power generator with intermediate loop

ABSTRACT

A thermoelectric power generator is disclosed for use to generate electrical power from heat, typically waste heat. An intermediate heat transfer loop forms a part of the system to permit added control and adjustability in the system. This allows the thermoelectric power generator to more effectively and efficiently generate power in the face of dynamically varying temperatures and heat flux conditions, such as where the heat source is the exhaust of an automobile, or any other heat source with dynamic temperature and heat flux conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to and claims the benefit of priority of U.S.Provisional Patent Application No. 60/694,746 entitled “High-EfficiencyThermoelectric Waste Energy Recovery System for Passenger VehicleApplications,” filed Jun. 28, 2005.

STATEMENT REGARDING FEDERALLY SPONSORED R&D

The U.S. Government may claim to have certain rights in this inventionor parts of this invention under Contract No. DE-FC26-04NT42279 awardedby the U.S. Department of Energy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application relates to the field of thermoelectric powergeneration, and more particularly to systems for improving thegeneration of power from thermoelectrics, particularly in the situationwhere there are limitations in the system on the temperaturedifferential across the thermoelectric.

2. Description of the Related Art

Thermoelectrics are solid state devices that operate to become cold onone side and hot on the other side when electrical current passesthrough. They can also generate power by maintaining a temperaturedifferential across the thermoelectric. Under many operating conditions,however, thermoelectric power generators are exposed to a combination ofchanging heat fluxes, hot side heat source temperatures, cold sidetemperatures, and other variable conditions. In addition, the deviceproperties, such as TE thermal conductance, figure of merit Z, heatexchanger performance all have a range that can combine to, in general,reduce device performance. As a result, performance varies and operationat a predetermined set point can lead to performance degradationcompared to the design.

Any process that consumes energy that is not 100% efficient generateswaste energy, often in the form of heat. For example, engines generate asubstantial amount of waste heat, representing inefficiency in theengine. Various ways to attempt to capture and use some of this wasteheat have been considered in order to improve the efficiency of any typeof engine, such as the engine in automobiles. Placing thermoelectrics onthe exhaust system of an automobile has been contemplated (See U.S. Pat.No. 6,986,247 entitled Thermoelectric Catalytic Power Generator withPreheat). However, the exhaust system varies greatly in temperatures andheat flux. Thus, because a thermoelectric generator is typicallydesigned to operate effectively over a small range of hot sidetemperatures, using exhaust for the hot side of a thermoelectricgenerator is suboptimal. In addition, a logical cold side coolant for athermoelectric generator linked to an engine system is the enginecoolant already provided. However, the coolant needs to be maintained ata fairly hot temperature for efficient engine operation. Thus, using theexisting coolant limits the temperature gradient that can be establishedacross the thermoelectric generator, thereby limiting effective wasteheat recovery.

SUMMARY OF THE INVENTION

Thermoelectric (TE) power generation from waste heat has suffered formany reasons in the past. The described invention addresses some ofthese delinquencies, greatly improving the capability of the TEgenerator. The improvement is generally obtained by providing anintermediate loop for the cool side, for the hot side or for both, andin one embodiment, providing advanced control for the intermediate loopor loops. Due to the great variability of the waste heat generated froman automobile engine, the present invention is presented in the contextof a thermoelectric generation system using waste heat from an engine ofan automobile as the thermal power source. This example permits aneffective disclosure of the features of the invention. However, thepresent invention is not limited to thermoelectric generators forautomobiles, or even for engines. The present invention has applicationin any thermoelectric generation system.

To effectively maximize the performance of a waste heat recovery systemusing thermoelectrics, maintaining the highest temperature differentialacross the thermoelectric generator module (TGM) is generally ideal. Oneway to do this is to keep the hot-side temperatures as high as possible.Another method is to better control the cold-side temperatures. By usingan intermediate heat transfer loop and appropriate control for that loopfor the hot side, the cold side, or both, significant improvements topower production and/or efficiency are obtained.

In the past (See U.S. Pat. No. 6,986,247), TE modules have been proposedas a lining around pipes or tubes carrying hot fluid, such as theexhaust system of an automobile. This provides intimate contact betweenthe hot fluid and the TE material, which is desirable to maximize thehot-side temperature of the TE material as well as the temperaturedifference across the TE material. This may be fine or even optimal fora static system where the hot fluid flow rates and temperatures do notchange. The TE elements can be designed to provide a perfect impedancematch with the hot fluid flow. This impedance match is important sinceelement geometry can greatly affect the amount of heat that can beeffectively transferred through the TE elements. For a static system,the thermoelectric system can be designed for steady state operation atmaximum efficiency or maximum power output or a combination acceptableto the designer.

However, once the system becomes dynamic, as in the case of anautomobile driven by an engine, where the range of temperatures and heatflux vary greatly, a thermoelectric generation system designed for aparticular set of conditions may only produce a small fraction of itscapacity, or even become negative under certain operating conditions. Inthe present invention, by providing a separate heat transfer loop forthe cold side, the hot side or both, and appropriate control for suchloop or loops, substantial improvements are obtained making such systemsfeasible in actual use.

One aspect of the present invention involves a thermoelectric powergeneration system. The system has a thermoelectric generator havingthermoelectrics with at least one cold side and at least one hot sideconfigured to generate electrical power when a temperature gradient ispresent between said at least one cold side and said at least one hotside. An intermediate heat transfer loop is in thermal communicationwith said at least one hot side and is in thermal communication with atleast one main heat source. A controller is in communication with flowcontrol devices in said intermediate heat transfer loop and is adaptedto control the heat flow in the intermediate heat transfer loop inresponse to changes in heat originating from the at least one main heatsource.

In one embodiment, the system further comprises thermal storage. Thethermal storage may be in the intermediate loop, associated with themain heat source or associated with both the intermediate loop and themain heat source. Advantageously, a heat exchanger between theintermediate heat transfer loop and the main heat source facilitates themovement of thermal power from the main heat source to the intermediateheat transfer loop. Preferably, a heat exchanger bypass is controllablevia the controller to cause some or all of the heat from the main heatsource to bypass the heat exchanger, depending on the heat from the mainheat source and the capacity of the thermoelectrics.

Another aspect of the present invention involves a thermoelectric powergeneration system for a main heat source. A thermoelectric generator hasthermoelectrics with at least one cold side and at least one hot sideconfigured to generate electrical power when a temperature gradient ispresent across said at least one cold side and said at least one hotside. An intermediate heat transfer loop is preferably in thermalcommunication with said at least one cold side and in communication withat least one heat dissipation device. Preferably, the heat dissipationdevice is separate from a main heat source. A controller incommunication with flow control devices in said intermediate heattransfer loop is adaptive to control the heat flow in the intermediateheat transfer loop in response to changes in operating conditions forthe thermoelectrics and/or the heat output from the main heat source.

In one embodiment, the heat source is an engine, having a main coolantsystem. Preferably, the intermediate heat transfer loop can be thermallyconnectable to the main coolant system of the engine, such that duringengine warm-up, heat transferred from the thermoelectrics to theintermediate heat transfer loop is further transferred to the maincooling system for the engine, thereby decreasing warm-up time for theengine. Further, a heat exchanger is advantageously provided between theintermediate heat transfer loop and the main heat source, with anoptional a heat exchanger bypass controllable via the controller, tocause some or all of the heat from the main heat source to bypass theheat exchanger.

Yet another aspect of the present invention involves a method ofgenerating power from waste heat from a main heat source using athermoelectric generator having thermoelectrics with at least one coldside and at least one hot side configured to generate electrical powerwhen a temperature gradient is present across said at least one coldside and said at least one hot side. The method involves transferringheat from the main heat source to an intermediate heat transfer loop,where the intermediate heat transfer loop is in thermal communicationwith a heat dissipation device. The flow of heat is controlled in theintermediate heat transfer loop in response to changes in operatingconditions of the main heat source.

In one embodiment, the heat source is an engine, having a main coolantsystem. In this embodiment, preferably, the intermediate heat transferloop can be thermally connectable to the main coolant system of theengine. Thereby, during engine warm-up, heat is transferred transferringfrom the thermoelectrics to the intermediate heat transfer loop andfurther to the main coolant system for the engine, thereby decreasingwarm-up time for the engine.

In one embodiment, the flow of heat from the main heat source to theintermediate loop is controlled based on changing heat flux of the mainheat source and the capacity of the thermoelectric generator.

Further aspects and features of the invention are disclosed inconnection with the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a thermoelectric generation system with a cold-sideintermediate heat transfer loop.

FIG. 2 illustrates another embodiment of a thermoelectric generationsystem with an cold-side intermediate heat transfer loop.

FIG. 3 illustrates a thermoelectric generation system with a hot-sideintermediate heat transfer loop.

FIG. 4 illustrates another embodiment of a thermoelectric generationsystem with a hot-side intermediate heat transfer loop, similar to thatdepicted in FIG. 3, but with added heat capacity storage.

FIG. 5 illustrates another embodiment of a thermoelectric generationsystem with a hot-side intermediate heat transfer loop, similar to thatdepicted in FIG. 3, but with added heat capacity storage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Automotive waste heat recovery is used as an example of the presentinvention. However, the invention is applicable to improve theperformance of power generation, waste heat recovery, cogeneration,power production augmentation, and other uses. As further examples, thepresent invention can be used to utilize waste heat in the enginecoolant, transmission oil, brakes, catalytic converters, and othersources in cars, trucks, busses, trains, aircraft and other vehicles.Similarly, waste heat from chemical processes, glass manufacture, cementmanufacture, and other industrial processes can be utilized. Othersources of waste heat such as from biowaste, trash incineration, burnoff from refuse dumps, oil well burn off, can be used. Power can beproduced from solar, nuclear, geothermal and other heat sources.Application to portable, primary, standby, emergency, remote, personaland other power production devices are also part of this invention. Inaddition, the present invention can be coupled to other devices incogeneration systems, such as photovoltaic, fuel cell, fuel cellreformers, nuclear, internal, external and catalytic combustors, andother advantageous cogeneration systems. It should also be understoodthat the number of TE modules described in any embodiment herein is notof any import, but is merely selected to illustrate the invention.

The present invention is introduced using examples and particularembodiments for descriptive and illustrative purposes. Although examplesare presented to show how various configurations can be employed toachieve the desired improvements, the particular embodiments are onlyillustrative and not intended in any way to restrict the inventionspresented. It should also be noted that the term thermoelectric orthermoelectric element as used herein can mean individual thermoelectricelements as well as a collection of elements or arrays of elements.Further, the term thermoelectric is not restrictive, but used to includethermoionic and all other solid-state cooling and heating devices. Inaddition, the terms hot and cool or cold are relative to each other anddo not indicate any particular temperature relative to room temperatureor the like. Finally, the term working fluid is not limited to a singlefluid, but can refer to one or more working fluids.

Cold Side Intermediate Loop

The cold-side temperatures that the theremoelectric generator module(TGM) are exposed to are often controlled by a cold-side loop. In thecontext of an engine, this cold-side loop can be the traditional vehicleradiator coolant loop, a stand-alone coolant loop or heat dissipator, ora combination of both. The radiator cooling loop has the advantage thatit is already available. Little additional design work may be needed.The capacity of the coolant loop would merely need to be sufficient tohandle the additional heat that would be dumped into it from the wasteheat transferred through the TGM.

The temperatures in a traditional radiator are maintained atapproximately 80°-110° C. These temperatures have been established basedon optimal engine performance and sufficient fan size. By dumpingadditional waste heat from a TGM system into the radiator coolant loopduring startup, substantial reductions in fuel consumption and emissionscan be achieved due to increased warm-up speeds for the engine andcatalytic converter. Additional waste heat in the main radiator coolingsystem also has the benefit in cooler weather of faster time topassenger compartment warmup, as well as improvement in warmup time ofother vehicle fluids, such as engine oil and other engine lubricationfluids. These are positive effects of using the traditional radiatorcoolant loop to maintain the cold-side of the TGM.

However, using the radiator as the cold side heat sink for the TGMlimits the ability to reduce the cold-side temperature below 80 Cwithout adversely affecting engine efficiency. Thus, in accordance withone aspect of the present invention, an intermediate or stand-alonecooling loop for the cold side of the TGM provides for possiblereduction in the cold-side temperature. This permits an increase in thetemperature differential across the TGM. Cooling for this intermediatecooling loop may be provided by an additional fan or a greater surfacearea heat sink. In one embodiment, the main chassis of the vehicle couldprovide a large heat sink.

In another embodiment, the intermediate loop is coupled or in thermalcommunication with the main coolant radiator during startup, anddecoupled for the remainder of operation. This would permit the fasterwarm-up times, yet permit maximizing the beneficial effects of thecold-side cooling system of the TGM. The embodiment described belowincorporates these features.

FIG. 1 schematically illustrates one embodiment of a thermoelectricgeneration system 100 in the context of using waste heat from an engineor any other heat source as the thermal power source for thethermoelectric generator module. The thermoelectric generation system100 has a thermoelectric generator module (TGM) 101, an auxiliarycooler, which can be any air-liquid heat exchanger known in the art,such as an auxiliary radiator 104, an auxiliary cold-side working fluidloop 118, an auxiliary pump or other device that controls fluid flowknown in the art 121, an auxiliary cold-side working (heat transfer)fluid 114, a bypass 120, and a hot side working fluid 107, such asexhaust, superheated steam, or any heat source. The hot side workingfluid 107 may also be an auxiliary heat transfer fluid in a separate hotside heat transfer loop, where the heat was obtained from the exhaust.This auxiliary heat transfer fluid will be discussed further relating toFIG. 3. The auxiliary cold-side heat transfer fluid 114 can be a liquidor molten salt such as liquid metal or NaK. It can also be a heattransfer fluid such as ethylene glycol/water or those made by Dow orother advantageous heat transfer fluids known in the art. It can also besuperheated or saturated steam or another type of two-phase fluid. Theauxiliary cold-side heat transfer fluid 114 can also be a gas likehelium, hydrogen, air, or carbon dioxide or any other high heat transfergas that can be operated at above atmospheric pressure in order toreduce pumping losses. The TGM 101 includes hot and cold side heatexchangers, TE elements, and electrical connectors between TE elements(not shown), preferably incorporating thermal isolation in the directionof working fluid flow, as described in U.S. Pat. No. 6,539,729, entitledEfficiency Thermoelectrics Utilizing Thermal Isolation, which patent ishereby incorporated by reference herein The TE elements in the TGM 304can be thermally isolated advanced high power density designs or can bemade of standard thermoelectric modules known in the art.

The thermoelectric generation system 100 is coupled at the cold side viaa heat exchanger (hex) 109 to a the cooling system for an engine or anyother heat source 102 having a main radiator 103, a first bypass 105, asecond bypass 119, a thermostat or valve or other fluid controlmechanism known in the art 106, a radiator valve or other fluid controlmechanism known in the art 112, main coolant 113, and a main pump 108.This connection to the cooling system of the engine 102 is optional. Byproviding the interconnection, faster warm-up times can be achieved forthe engine, passenger compartment, and the like, during engine warm up.It will then be appreciated that the intermediate cold-side workingfluid loop 118 may be completely uncoupled from the engine coolingsystem, and provide benefits of the present invention of an intermediatecold-side heat transfer loop, not limited by the operating restrictionson the coolant system for the engine.

A controller 150 monitors and controls operation. A number of sensorsare advantageously strategically placed to monitor the system. Thesesensors are preferably temperature and/or flow sensors. The controllerthen communicates with control mechanisms such as the pumps 108, 121 andvalves 110, 112. The particular connections shown are merely exemplary,and sensors and control connections are provided as appropriate for thesystem design.

During steady state operation, main coolant 113 is circulated throughthe main coolant loop 117 using main pump 108. The main coolant 113 isreturned to the engine 102 after it has gone through the main radiator103 and been cooled by the airflow 116, which can be ram or fan air,flowing across the main radiator 103 in a typical cross-flow heatexchanger. This is standard operation for a vehicle cooling system.During vehicle startup, it is also standard for the thermostat 106 toprevent main cold flow 113 through the main radiator 103, directing itthrough the bypass 105 and back to the engine 102. This allows the maincoolant 113, and thus the engine 102, to warm up faster. Engines aredesigned to run at higher efficiencies once they are warm. In addition,the catalytic converter in the exhaust system of the vehicle, whichhelps reduce harmful emissions, does not start being effective until itsinternal temperature reaches a specific “light off” temperature for itsinternal catalysts. Thus, it is possible to reduce emissions andincrease vehicle fuel economy by warming up the engine faster.

The thermoelectric generator module (TGM) 101 is the component in thethermoelectric generation system 100 that generates electrical powerfrom the waste heat of the vehicle. The TGM 101 can operate moreefficiently if its cold-side temperature is kept as cold as possible.The main coolant 113 must operate between 80-110° C. to allow the engineto operate properly. This is the case no matter what the ambienttemperature outside. The TGM 101 operates more effectively at a lowercold-side temperature than this. Thus, the TGM 101 is connected to theauxiliary cold-side heat transfer loop 118 using the auxiliary radiator104. The auxiliary heat transfer fluid 114 is pumped through theauxiliary radiator 104 and back to the TGM 101 with the auxiliary pump121. Airflow 115, which can be ram or fan air, flows across theauxiliary radiator 104, which may also be a cross-flow heat exchanger,removing heat from the auxiliary cold side working fluid 114. Theauxiliary coolant 114 is now independent from the main coolant 113, andthus can be controlled by a separate pump and maintained at atemperature closer to ambient.

In one embodiment, to maximize performance throughout the drive cycleand take as much advantage of the waste heat as possible, the maincoolant loop 117 and the auxiliary coolant loop 118 are connected withan optional heat exchanger 109. During vehicle startup, valve 112 isopen allowing main coolant 113 to flow through heat exchanger 109. Valve110 is also open allowing auxiliary coolant 114 to flow through heatexchanger 109 transferring waste heat from the TGM 101 stored in theauxiliary cold fluid 114 to the main coolant 113. This allows the engineand catalytic converter to warm up faster providing the benefitsdescribed above. It will be understood that the heat exchanger 109 couldsimply interconnect the two cooling loops 117, 118, or could be a heatexchanger which facilitates heat transfer between the main coolant 113and the auxiliary coolant 114.

Once the engine 102 reaches operating temperature, and the catalyticconverter (not shown) has reached “light off” temperature, sensors (notshown), which may exist in the main coolant loop 117, intermediatecoolant loop 118, engine 102, and the catalytic converter (not shown),communicate this information to controller 150. Controller 150 thencloses valves 110 and 112 preventing main coolant 113 and auxiliary heattransfer fluid 114 from going through heat exchanger 109. In thisembodiment, this effectively isolates the two systems. Controller 150may also control pump speed for both auxiliary pump 121 and/or main pump108. Main coolant 113 circulates through bypass 119 and auxiliary coldfluid travels through bypass 120. The main coolant loop 117 can operateat one temperature and the auxiliary heat transfer loop 118 can operateat another, preferably lower, temperature. Hot side fluid 107 flowsthrough the TGM 101 to provide heat and higher temperature for the hotside of the TGM 101.

As briefly mentioned above, the hot-side fluid 107 may be exhaust fromthe engine 102, or may, in a preferred embodiment, be a separatehot-side heat transfer fluid, as will be explained further herein. Mainpump 108 may be a similar or different device from that of auxiliarypump 121. Similarly, main valve 112 may be similar or different toauxiliary valve 110. Main radiator 103 may be similar or different fromauxiliary radiator 104. Main coolant fluid 113 may be similar ordifferent from auxiliary coolant 114.

FIG. 2 shows another embodiment for a thermoelectric generation system200, similar in many respects to that of the embodiment to FIG. 1. Anengine 102, such as an engine in a car, provides a source of heat. Theengine 102 has a cooling system using a radiator 103. A separateintermediate cold side loop uses a heat dissipater 204 instead of theauxiliary radiator 104 and the airflow 115 is removed (FIG. 1).

The heat dissipater 204 is representative of any other type of heatexchanger. For example, it could be the chassis of the vehicle. Anauxiliary cold fluid 211 still flows through the heat dissipater 204 asdid auxiliary cold-side working fluid 214 through the auxiliary radiator104 (FIG. 1). The heat transfer mechanism in this embodiment preferablyis dominated by a large surface area and convection as opposed to a morecompact heat transfer surface area and forced air convection as waspresented with the auxiliary radiator 104 in FIG. 1.

Hot Side Intermediate Loop

As with the cold-side intermediate loop, the present invention addressesthe potential impedance mismatch for dynamic systems through anintermediate control loop for the hot-side of the TGM as well. In thisembodiment, like the cold-side intermediate loop, the flow rate andtemperature of the fluid can be adjusted to better match that needed toprovide maximum power output and/or improved efficiency for a dynamicset of operating conditions. Thus, the power generator performs closerto optimal over a wider range of operating conditions.

The ability to control the flow rate in the intermediate loop controlsthe heat flux across the TE elements, where this is not possible insystems proposed in a conventional design. It also allows for moreoptimal thermodynamic cycles, such as thermal isolation in the directionof flow (as described in U.S. Pat. No. 6,539,729), to be implemented.

Thermal mass can also be provided in the intermediate loop sufficient toenable the power output of the generator system to be more constant overan entire dynamic cycle by storing heat energy during periods of highheat flux and releasing energy during periods of low heat flux.Controlling the pump speed can also aid in providing a more constantlevel of power. This ability to provide a constant thermal power levelover a highly dynamic range of conditions can greatly simplify powerconversion and controls for the generator system.

By incorporating the intermediate loop, the TE generator can also beisolated from the hot-side system, which is often operating under harshconditions. This may be desirable in many instances, in particular wheremaintenance may be necessary on one system but it is undesirable todisturb the other system. For example, the intermediate loop allows theTE portion of the TE generator system to be contained within a separatehermetically sealed package. This can also allow for easier recycling ofthe TE material.

The intermediate loop also provides the ability to choose a better heattransfer fluid than the main heat source. If an intermediate loop fluidis chosen with better heat transfer characteristics than the primary hotfluid (e.g., engine exhaust), the thermoelectric generator module (TGM)can be built smaller. This compact size improves the ruggedness of thedevice and can enable it to fit applications where size, weight, andcost are critical.

The intermediate loop also permits the working fluid to be selected tobe stable over the entire exposed temperature range. A working fluidhaving excellent heat transfer-related thermodynamic properties willminimize the amount of heat loss associated with the additional loop.Pump losses associated with moving the fluid through the control loopshould be small in order to not offset the power generated by the TGM.

By having an intermediate loop, the possible array of working heattransfer fluids is greatly expanded. Liquid metals such as gallium andNaK have excellent thermodynamic properties and are stable liquids overa wide temperature range. As liquids, they also would help keep pumpinglosses at a minimum. However, they have very poor material compatibilitywith a variety of different materials, and, thus, are not necessarilygood choices for this application. But, the existence of theintermediate loop provides the option to consider such fluids in thepossible working fluids.

Other heat transfer liquids considered that are manufactured by suchcompanies as Dow do not remain stable over certain temperatures. Evensilicone liquids become unstable over 400° C.

For the present applications, the fluids that remain stable over widetemperature ranges are generally gases. Gases such as hydrogen andhelium have excellent thermodynamic properties. Hydrogen, unfortunately,is a highly flammable fluid and can cause materials to become brittle,particularly at high temperature. Helium, however, is inert and, thus,has excellent material compatibility. It has excellent thermodynamicproperties when compared to water. In some respects it has betterthermodynamic properties than glycol solutions. Its main drawback is itsdensity. Being a very light gas, at atmospheric pressure, pumping lossesare high for helium. These pumping losses, however, can be minimized byincreasing the fluid's working pressure as well as mixing the heliumwith a small amount of a heavier fluid. One such fluid is xenon, whichis also an inert gas and is substantially heavier than helium. Xenondoes not have good thermodynamic properties and may be more expensivethan helium. Thus, in one embodiment, the amount of xenon used would beminimized.

There are other gas possibilities for the working fluid. These includeCO₂ and air. Both of these gases are heavier than helium and, thus, donot require the addition of xenon, but they have worse thermodynamicproperties.

FIG. 3 shows the first embodiment of the power generator system 300 withintermediate loop control 311. A main hot fluid 301, which could bevehicle exhaust gas, superheated steam, or any heat source, flowsthrough the primary heat exchanger (PHX) 303. The PHX 303 can be a shelland tube or any other heat exchanger type known in the art. If thetemperature or mass flow rate of the main hot fluid 301 exceeds presetlimits, a 3-way valve 308 or other flow directional device known in theart can be opened such that flow will be directed through the bypass 307rather than through the PHX 303.

Preferably, a control system monitors the temperature and a controller350 for the central system and adjusts the valve 308 to the correctlevel so that the system operates as effectively as practical. Thishelps to protect the thermoelectric (TE) material in the thermoelectricgenerator module (TGM) 304 from being overheated. In the example of anengine, this can also be used to prevent excessive backpressure in themain hot-side loop 309 (e.g., in the exhaust), which in the case of amotor vehicle can adversely affect the performance of the vehicle'sengine. Preferably, the bypass 307 routes the main hot fluid (some smallportion to virtually all of the fluid) around the PHX 303 and back tothe main hot-side loop 309, when that improves performance or achievessome other desired result. The valve 308 thus allows partial flow to gothrough the bypass 307 and partial flow through the PHX 303 under thecontrol of the controller 350. Preferably, the controller 350 hassensors at for at least the hot fluid 301, an intermediate loop fluid302 and a cold-side working fluid 305. In one embodiment, the controllersensors would detect at least temperature, and possibly flow rate forthe fluids. Also, the controller 350 preferably provides control for thevalve 308 and pump 306, to control the flow rates and proportion theflow through valve properly.

In this embodiment, heat is transferred from the main hot fluid 301 tothe intermediate loop hot-side working fluid 302 via the primary heatexchanger PHX 303. In one embodiment, the intermediate loop hot-sideworking fluid 302 can be a liquid or molten salt such as liquid metal orNaK. It can also be a high temperature heat transfer fluid such as thosemade by Dow. The intermediate fluid 302 can be superheated or saturatedsteam or another type of two-phase fluid. The intermediate fluid 302 canalso be a gas like helium, hydrogen, air, or carbon dioxide or any otherhigh heat transfer gas that can be operated at above atmosphericpressure in order to reduce pumping losses.

A pump 306 or other device capable of controlling flow known in the art306 can control the thermal mass flow or thermal impedance of theintermediate loop fluid 302 to “match” or equal that of the hot fluid301, if desired. This helps to maximize the effectiveness of the PHX303. This is particularly important for a system where the hot fluid 301flow rate or temperature fluctuates over a wide range. Maximizing theeffectiveness of the PHX 303 over the dynamic range of hot fluid flowsimproves TGM 304 and thus the entire generator system 300 over an entirecycle. Without this control, the system would only operate at optimalperformance over a very narrow range of operating hot fluid 301 flows.The intermediate loop 309 also provides a level of thermal storage forthe system. The thermal mass of the intermediate loop fluid 302 providesa means of thermal energy storage, which can be augmented by the pump306 flow speed.

The intermediate loop heat transfer fluid 302 is circulated around thehot-side intermediate loop 311 and through the TGM 304 using the pump306. The TGM 304 preferably includes hot and cold side heat exchangers,TE elements, and electrical connectors between TE elements (not shown),preferably incorporating thermal isolation in the direction of workingfluid flow, as described in U.S. Pat. No. 6,539,729, entitled EfficiencyThermoelectrics Utilizing Thermal Isolation. The TE elements in the TGM304 can be thermally isolated advanced high power density designs or canbe made of standard thermoelectric modules known in the art. Cold-sidefluid 305 flows through the cold-side heat exchanger portion of the TGM304 to complete the thermoelectric generator.

FIG. 4 illustrates a thermoelectric generation system 400 very similarto that of the system 300 of FIG. 3. This thermoelectric generationsystem 400 includes an additional thermal storage 409. Controller 350preferably includes an added sensor for the thermal storage 409. Thesensor permits the controller 350 to calculate whether additionalthermal storage capacity is available, and effectively use this storagein controlling the operation of the system 400. This thermal storage 409can be any type of media with thermal mass, including phase changematerial and the like. This thermal storage 409 enhances the thermalstorage capacity of the intermediate loop 311 allowing the system 400 toproduce useful power during periods of low temperatures and mass flowsof the hot fluid 301 up to and including even when there is no hot fluid301 flow. When there is no hot fluid flow, the system would then“borrow” heat from the thermal storage. Likewise, when there is anexcess of thermal power above that which the system 400 is capable ofutilizing efficiently, thermal power can be stored.

FIG. 5 shows a system 500 that is similar to that of the system 400 ofFIG. 4. The difference is that the thermal storage 509 is located in thehot-side loop 310 rather than the intermediate loop 311. Again, thecontroller 350 preferably includes a sensor for the thermal storage 509,to detect the temperature of the thermal storage 409, under the controlof controller 350. The thermal storage 509 at this location in thisembodiment is advantageous if there is space (volume), weight, or otherconsiderations that would prevent the thermal storage from being locatedas it is in FIG. 4.

1. A thermoelectric power generation system comprising: a thermoelectricgenerator having thermoelectrics with at least one cold side and atleast one hot side configured to generate electrical power when atemperature gradient is present between said at least one cold side andsaid at least one hot side; an intermediate heat transfer loop inthermal communication with said at least one hot side and in thermalcommunication with at least one main heat source, the at least one mainheat source having a first working fluid flowing through the at leastone main heat source, the intermediate heat transfer loop having asecond working fluid flowing through the intermediate heat transfer loopand in thermal communication with said at least one hot side, whereinthe intermediate heat transfer loop and the at least one main heatsource are not in fluid communication with one another; and a controllerin communication with flow control devices in said intermediate heattransfer loop, the controller adapted to control the heat flow in theintermediate heat transfer loop in response to changes in heatoriginating from the at least one main heat source.
 2. Thethermoelectric power generation system of claim 1, further comprising athermal storage.
 3. The thermoelectric power generation system of claim1, further comprising a heat exchanger between the intermediate heattransfer loop and the main heat source, and further comprising a heatexchanger bypass controllable via the controller, to cause some or allof the heat from the main heat source to bypass the heat exchanger.
 4. Athermoelectric power generation system for a main heat source, the powergeneration system comprising: a thermoelectric generator havingthermoelectrics with at least one cold side and at least one hot sideconfigured to generate electrical power when a temperature gradient ispresent across said at least one cold side and said at least one hotside; an intermediate heat transfer loop in thermal communication withsaid at least one cold side and in communication with at least one heatdissipation device, wherein the heat dissipation device is separate froma main heat source, the main heat source having a first working fluidflowing through the main heat source, the intermediate heat transferloop having a second working fluid flowing through the intermediate heattransfer loop and in thermal communication with said at least one coldside, wherein the intermediate heat transfer loop and the main heatsource are not in fluid communication with one another; and a controllerin communication with flow control devices in said intermediate heattransfer loop, the controller adapted to control the heat flow in theintermediate heat transfer loop in response to changes in operatingconditions for the thermoelectrics.
 5. The thermoelectric powergeneration system of claim 4, wherein the heat source is an engine,having a main coolant system.
 6. The thermoelectric power generationsystem of claim 5, wherein the intermediate heat transfer loop can bethermally connectable to the main coolant system of the engine, suchthat during engine warm-up, heat transferred from the thermoelectrics tothe intermediate heat transfer loop is further transferred to the maincooling system for the engine, thereby decreasing warm-up time for theengine.
 7. The thermoelectric power generation system of claim 4,further comprising a heat exchanger between the intermediate heattransfer loop and the main heat source, and further comprising a heatexchanger bypass controllable via the controller, to cause some or allof the heat from the main heat source to bypass the heat exchanger.
 8. Amethod of generating power from waste heat from a main heat source usinga thermoelectric generator having thermoelectrics with at least one coldside and at least one hot side configured to generate electrical powerwhen a temperature gradient is present across said at least one coldside and said at least one hot side, the method comprising the steps of:transferring heat from a first working fluid flowing through the mainheat source to a second working fluid flowing through an intermediateheat transfer loop and in thermal communication with said at least onecold side or with said at least one hot side, wherein the intermediateheat transfer loop is not in fluid communication with the main heatsource, the intermediate heat transfer loop in thermal communicationwith a heat dissipation device; and controlling the flow of heat in saidintermediate heat transfer loop in response to changes in operatingconditions of the main heat source.
 9. The method of claim 8, whereinthe heat source is an engine, having a main coolant system.
 10. Themethod of claim 9, wherein the intermediate heat transfer loop can bethermally connectable to the main coolant system of the engine, themethod further comprising the step of during engine warm-up,transferring heat from the thermoelectrics to the intermediate heattransfer loop and further to the main coolant system for the engine,thereby decreasing warm-up time for the engine.
 11. The method of claim8, further comprising the step of controlling the flow of heat from themain heat source to the intermediate loop based on changing heat flux ofthe main heat source and the capacity of the thermoelectric generator.12. The thermoelectric power generation system of claim 1, furthercomprising a heat exchanger in thermal communication with theintermediate heat transfer loop and in thermal communication with themain heat source.
 13. The thermoelectric power generation system ofclaim 12, further comprising thermal storage.
 14. The thermoelectricpower generation system of claim 12, further comprising a heat exchangerbypass controllable via the controller to cause some or all of the heatfrom the main heat source to bypass the heat exchanger.
 15. Thethermoelectric power generation system of claim 4, wherein the heatsource is an engine having a main coolant system.
 16. The thermoelectricpower generation system of claim 15, wherein the intermediate heattransfer loop is thermally connectable to the main coolant system of theengine, such that during engine warm-up, heat transferred from thethermoelectrics to the intermediate heat transfer loop is furthertransferred to the main cooling system for the engine, therebydecreasing warm-up time for the engine.
 17. The thermoelectric powergeneration system of claim 4, further comprising a heat exchanger inthermal communication with the intermediate heat transfer loop and inthermal communication with the main heat source.
 18. The thermoelectricpower generation system of claim 17, further comprising a heat exchangerbypass controllable via the controller to cause some or all of the heatfrom the main heat source to bypass the heat exchanger.
 19. Thethermoelectric power generation system of claim 1, wherein the firstworking fluid and the second working fluid are different from oneanother.
 20. The thermoelectric power generation system of claim 4,wherein the first working fluid and the second working fluid aredifferent from one another.
 21. The method of claim 8, wherein the firstworking fluid and the second working fluid are different from oneanother.